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1974 - Kieke & Hartmann - Detecting Microporosity to Improve Formation Evaluation

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Detecting Microporosity to Improve Formation Evaluation
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  +“4S 3/DetectingMicroporosityToImproveFormationEvaluation E.M.Kieke, SPE-AIME,AmocoProductionCo,D. J.I-Iartmann, SPE-AIME,AmocoProductionCo. Introduction Evaluationwiththemostrecentlydevelopedloggingtoolshasrevealedanomalousconditionsofhighwatersaturationinproductiveintervalsandhasindicatedtheneedforasupplementalevaluationtoolortech-niqueforsuchcircumstances.Theanomalieswereobservedinbothelasticsandcarbonateswerabroadgeographicarea.Thescanningelectronmicroscope(SEM)hasbeenusefulfor(1)grainsurfacestudiesrelatedtoenviron-mentsofdeposition,]”(2)microfossilinvestigationforbiostratigraphjcdatil~gandalterationofsedi-ments,z’3and(3)documentationofmicroporosityincarbonaterocks.sUseoftheSEMhasimprovedourunderstandingoftheactualporeconfigurationofreservoirrocks—characteristicsthatcanbeobservedonlyathighmagnification,CoreandcuttingsamplesfromnumerouswellshavebeeninspectedwiththeSEM,Observationsindicatethatwherethereservoirruckcontainsmicro-porosity,theupperlimitsforwatersaturationdeter-minedbyloganalysiscanbeincreased.Theminimumwatersaturationdependsinpartontherelativeamountofmicroporosityintherock. DetectionofMicroporosity Reservoirrocksaredividedintotwobasiccategories:(1)elastics,whichincludeslits,sands,andgravel;and(2)carbonates,whichencompasslimestonesanddolo-mites.Theporespacebetweensandorcarbonategrainsasoriginallydepositediscalledintergranularporosity,Vugularporosity,anotherwellknowntype,canbeformedbyleachingofcarbonategrainsorothersolub;ematerial,whichresultsinporeslargerthanthegrainsizeoftherocks.Bothintergrainandvugularporositycanbedetectedvisuallyoratlowmagnifica-tion(50xorless).Inspectionofbothclasticandcar-bonatesampleswiththeSEMcanrevealanothertypeofporosity:micropcmosity,Nolimitsonphysicalmeasurementshaveyetbeenestablishedtodefineamicropore.Rather,ithasbeenacceptedthatamicro-poreissosmallthatitcannotbeseenatmagnifica-tionsoflessthan50xandrequires100Xto5,000Xtobestudiedindetail.Thepresenceofmicroporosityinareservoirincreasesthecapillaryattractiontowater,frequentlyresultinginveryhighimmobilewatersaturation.Themicroporosityisformedbyoneofseveralmethods,includingrecrystallizationofgrainsandoolites,dolomitization,formationofinter-crystallinepores,andsecondarycementation.Thusfar,nocompletelyreliablemethodhasbeenestab-lishedforrecognizingreservoirscontainhgmicro-porosityexceptinspectionofsampleswithanSEM.Severaltechniqueshave,however,offeredvaluableassistanceinrevealingsuchreservoirs.Theuseofmudloggersduringdrillinghasinsomecasesrevealedshowswherelogcalculationsindicatedhighwatersaturation.Subsequenttestinghassometimesyieldedhydrocarbonproductionwhereexaminationofsam-plesrevealedmicroporosity.Theuseofspeciallog- ljyoucan’tseethetreesforthejorest getascanningelectronmicroscope Itcangetyoucloseenoughtodiscoverpreciselywhatconditionsare andinconjunctionwithotherdevicesmaykeepyoujromturningdownsomepromisingreservoirs  gingdevicestodeterminesaturationintheflushedzonehasalsobeenofvalueinsuggestingthepresenceofmovablehydrocarbonsinmembersforwhichhighwatersaturationhasbeencalculated, SomeTypesofPorosityinCarbonateReservoirsIntergrainandVu@arPorosity Figs.1aand1bareSEMphotographofsamplesfromtheSmackoverformationinLewiwilletield,LafayetteCounty,Ark.Themagnificationisonly10X.Asthesepicturesindicate,thereisadifferenceinporegeometrywithinthereservoiroverashortverticaldistance.Thevari-ationsarerepeatedastheentireSmackoverformationispenetratedinthiswell.Fig.1aillustratesthelimestonegrainsoftheforma-tionthatformtheintergranularporosity.Theporesareofuniformdiameterandhavelarge,openporethroats.Fig,1bshowsthevugsformedbyremovalofthegrains.Areductionofintergrainporosityandper-meabilitywascausedbyinfillingofdolomitecrystals,followedby:eachingofthegrains.Therewasare-sultingincreaseintotalporosity,butpermeabilityisstillcontrolledbythecrystallinecement. MicroporosityOnly. Figs.2aand2bshowarock ?7 7.$,t.,*... 4, .’j;~?t~  ’ .’”i. .  II’~<... a.  --4 looopb.Fig. l—Smackoverformation, Lewisvillefield,Lafayette County, Ark.(1OX), OCTOBER,1974 whose poresystemconsistsentirely of microporosity. ThesampleisfromanooliticSmackoverlimestoueinMarionCounty,Tex.Thelimestone~asbeencom-pletelycementedwithcalcitecement,Fig.2ashowssomemicroporosityinthelimestonegrainaswellasinthecementbindingthegrainstogether.Thediam-eterofthemicroportxinthecementislargerthanthemicroporosityofthegrains,butitstillfallsinthemicroporosityrange.Theporesarenotvisiblewithnormalmagnification,whichwouldcauseonetoquestion10-percentlogpor,@tyiftheSEMphoto-graphs were notavailable.Fig.2b,at5,000Xmag-nification,showstheinterconnectionofthemicro-pores,aswellasthecalcitecrystalsthatmakeuptheoolites,andsubstantiatestheporositycomputedbylogs.Thegrainsapparentlywereanunstableformofcalciumcarbonate,andafterdepositiontheyre-crystallizedintoamorestableform.Lessvolumewasrequired,resultinginthemicropores.Experiencetodateindicatesthatwhenhydrocarbonsoccurinsuchasystemofmicroporosity,theycanbedispersedthrougheachporeinassociationwithmovabIewater,dependingonthelengthofthehydrocarboncolumn,Torecoverthehydrocarbons,someofthewatermustalsoberecovered.Inacompletemicroporesystemthesmallsizeoftheporethroatscausesthecapillaryforcestobehighanddemandsahighpressurediffer-entialtoproducethereservoirtluids,Thus,ifonlymicroporosityexistsinthebedandthereisagas/wateroroil/watercontact,thepossibilitythatwaterwillconeintothewellboreishigh. CombinationMicroporosityandMacroporosity TheSEMphotographsinFig,3showexamplesof MICROPOREa.Porosityincement,microporosityinlime.stonegrain (500X). mMicroporositythatappearstopermeateentiregrain(5,000X).Fig.2—Smackoverformation,MarionCounty,Tex.1081  bothintergrain,ormacroporosity,andmicroporosityintheRodessalimestoneofHoustonCounty,Tex,Thesamplewasobtainedfromtheupperportionofthepayzone.Fig.3a,at20X,indicatessomevisibleporosity.Theblackareasareintergrainporositywithoccasionalvugs,Fig.3b,at500X,showsthecoarsesurfaceofthegrainsandthedensenessofthecement-ingmaterialfillingthespacebetweenthegrains,Onlyasmallamountofmicroporositywasnotedinthecementingmaterial.Figs.4aand4barephotographsofasampleobtainednearthebaseofthesamepaysection.Fig.4ashowsthatatlowmagnificationthesampleappearstohavegoodintergrainporosity,Fig.4b,at500x,showsmicroporosityinthefirstlayeraroundthegrainsandthroughoutthegrains.Thiscanallowhydrocarbonstomcveintoandoutoftheintergrainporeswithoutmovingthewaterinthe micropores. SomeTypesofPorosityinChstics Sandstonesconstitutealargeportionofourreservoirrockandhavegenerallybeenregardedaseasytoanalyze.InspectionofsampleswiththeSEMrevealsthatsandstonescanpossessthesamebasictypesofporegeometrythatoccurincarbonates. IntergrainPorosity. Figs,5aand5billustratearela-tiw:lycleanFriosandfromMatagordaCounty,Tex.Thesandcontainsintergrainporositywithsomesec-ondaryoccurrenceofclaymaterial,identifiedasaclaybridge.Inthisparticularcase,thereisonlyasmallamountofclaymaterialintherockanditseffectonpermeabilityisminor,Fig,5bisanenlargedphotographoftheclaybridge. —. Soojl — a.Lowvisualporosity—dense,wellcemented(20X), — 2op—b.Unalteredcoarsesurfaceofpelletsanddensecementfillingtheporas(500X).Fig.3—Rodessalimestone,HoustonCounty,Tex.1082a,Goodvisualporosity,verylittlecementation(20X).b.Microporosityinfirstlayeraroundpellet(500X).Fig. 4-Rodessalimestone,HoustonCounty,Tex.JOURNALOFPETROLEUMTECHNOLOGY  VugularPorosity. Somefeldsparwaspresentintldssample(Fig,5a).Thecrystaloffeldspar,whichisidentifiedinthepicture,isconsiderablylargerthanthesandgrainsandhasalreadybeenpartiallyleachedout,Once~niscrystaliscompletelyleachedout,thevugthatiscreatedwillbeconsiderablylargerthantheintergraina?eas.Althoughthisisnotcommoninelastics,itisbeingrecognizedmorefrequently. Microporosity. Manysandshavebecomeverypoorreservoirrocksbecausesome claycement hasformedintheintergranularpores.Asandthatoriginallyre-sembledthatinFig.5becomesliketheoneinFig.6,alsoaFriosandfromMatagordaCounty,Tex.Thisisafine-grainedsandwithsomemacroporosity(blackareas)asshowninFig,6a(50X).Muchoftheoriginalporosityisfilledwithacementingma-terial.Asthesamplewasinspectedunderincreasingmagnification,itwasseenthatthelargeporesarepartiallyfilledwithcalcitecrystalsand“honeycomb”claycement.Theclayhasmicroporosity,whichisrevealedinFig.6b.Themicroporesintheclaycanholdirreduciblewaterwhileproduciblehydrocarbonscanbecontainedinthemacropores.Thl?fillingofthemacroporositywithclayreducestheporosityonlyslightly,butgreatlyreducesthepermeability.Thisisbecausethehighporositywithintheclaycon-sistsofmicroporesthatarenutinterconnectedandbecausetheclaytendstogrowintothethroatsofthemacropores.Anotherformofsecondarycementingmaterialis quartz. Fig.7aisan SEMph@ wh ofaFriosandinMatagordaCounty,Tex.;itshowsseveralsagdgrainsoutliningaporepartiallyfilledbyanangularquartzcrystal.Theporosityrapidlydecreasedwithdepositionofquartzcementinthemacroporebe-causethequartzcrystaldoesnotcontainanymicro-porosity,butformsmicroporesbetweenthecrystalfaces.Quartzcementalsoreducedpermeability,but notso adverselyasclaycementingmaterial.Asmorecrystalsgrowintothepore,thequartzwillfillthespaceandcementthesendintoaveryhard,denserock.Frequently,asandwithsomequartzcementalsocentsinsenough daycement toseriouslyaffectper-meability.Fig.7bshowsaquartzcrystalandmicro-porousclayfillingaportionofthemacroporearea.FormationwatersaturatedwithCaCO,passing ~hrough r s rvoir ock supplies the calcitecement notsdinsomesands.Itisnotsoccmmonasclayorquartzcement;however,calciteoccursinmanyreser-voirs,formingmicroporesbetweenthecrystalfacesinmuchthesamewaythatqtiartzcementdoes. EffectofMicroporosityonLogResponse Documentationofthegeomet~ofporosityandits a,Clean,wellsorted,medium.grainedsandstonewithsomesecondarycement(50X).a.Quartzgrainswithhighvolumeofcc(50X). — 20 .—CALCITECRYSTALb.MicroporositY(1,000X).Fig.6-Friosand,MatagordaCounty,Tex.Porosity,18to22percent;permeability,10to80md”1083
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